The Independent National Electoral Commission posted on its website the count from 28 of the country's 36 states and its capital, showing Jonathan with more than 19 million votes, compared to his main challenger -- Muhammadu Buhari -- who had close to 9 million votes.

A formal announcement of the results could come as early as Monday.

To avoid a runoff, Jonathan must get at least a quarter of the vote in two-thirds of the 36 states and the capital.

Nigerians voted Saturday for their president, a week after parliamentary elections were marred by violence and accusations of fraud in Africa's most populous nation.

Jonathan is the front-runner despite a poor performance in those elections by his People's Democratic Party. He is popular in the Christian and animist south.

The former vice president assumed office after President Umaru Yar'Adua died last year following treatment for a kidney ailment in Saudi Arabia.

Jonathan has led the nation of about 150 million people since May. About 73 million people were registered to vote.

His main challenger, Buhari, is a former military ruler and was a contestant in the 2003 and 2007 elections. He is the candidate for the Congress for Progressive Change and enjoys support from the mostly Muslim north.

Other candidates included Nuhu Ribadu and current Kano state Gov. Ibrahim Shekarau.

A CNN iReporter in Lagos, who gave her name as Jan Young, said Saturday she expected the race to be tight.

"If the incumbent president wins, it won't be a landslide victory but a fair split between the ruling and opposition parties who campaigned for our votes. I also expect our nation would demand accountability from whoever wins at the end of the day," she wrote.

Saturday's voting was largely peaceful, in contrast to the violence that characterized the country's parliamentary elections on April 9. During that vote, separate bomb blasts ripped through a polling station and a collation center in northeastern Nigeria.

Human Rights Watch has estimated that at least 85 people have been killed in political violence so far.

A new election chief promised free and fair elections, but the electoral commission was forced to put off elections earlier this year by a week after logistical problems, including party logos missing from ballot papers, were reported nationwide.

It was a major setback reminiscent of the nation's 2007 elections, which the European Union described as filled with rampant vote rigging, violence, theft of ballot boxes and intimidation.

Nigeria, Africa's most populous country and its largest oil producer, is a major supplier of crude oil to the United States, and hosts many Western oil companies and workers.

Nigerians voted April 9 for 360 House of Representatives seats and 109 Senate seats. A gubernatorial vote will be held on April 26.